Toán Lớp 8: Tìm x
1. (4x -3)^2 – 3x(3 – 4x) = 0
2. 3x (x – 4) – x(5+3x) = -34
3. x^3 – 9x – 5x^2 + 45 = 0
4. x^3 – 4x^2 – 9x= -36
5. x^2 – 7x + 6 = 0
6. 8x^3 -12x^2 + 6x -1 = 0
7. x^2 + 5x + 4 = 0
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Toán Lớp 8: Tìm x
1. (4x -3)^2 – 3x(3 – 4x) = 0
2. 3x (x – 4) – x(5+3x) = -34
3. x^3 – 9x – 5x^2 + 45 = 0
4. x^3 – 4x^2 – 9x= -36
5. x^2 – 7x + 6 = 0
6. 8x^3 -12x^2 + 6x -1 = 0
7. x^2 + 5x + 4 = 0
Comments ( 1 )
1){\left( {4x – 3} \right)^2} – 3x\left( {3 – 4x} \right) = 0\\
\Leftrightarrow \left( {4x – 3} \right).\left( {4x – 3 + 3x} \right) = 0\\
\Leftrightarrow \left( {4x – 3} \right).\left( {7x – 3} \right) = 0\\
\Leftrightarrow \left[ \begin{array}{l}
x = \dfrac{3}{4}\\
x = \dfrac{3}{7}
\end{array} \right.\\
Vậy\,x = \dfrac{3}{4};x = \dfrac{3}{7}\\
2)3x\left( {x – 4} \right) – x\left( {5 + 3x} \right) = – 34\\
\Leftrightarrow 3{x^2} – 12x – 5x – 3{x^2} = – 34\\
\Leftrightarrow – 17x = – 34\\
\Leftrightarrow x = 2\\
Vậy\,x = 2\\
3){x^3} – 9x – 5{x^2} + 45 = 0\\
\Leftrightarrow x\left( {{x^2} – 9} \right) – 5\left( {{x^2} – 9} \right) = 0\\
\Leftrightarrow \left( {{x^2} – 9} \right)\left( {x – 5} \right) = 0\\
\Leftrightarrow \left( {x + 3} \right)\left( {x – 3} \right)\left( {x – 5} \right) = 0\\
\Leftrightarrow x = – 3;x = 3;x = 5\\
Vậy\,x = – 3;x = 3;x = 5\\
4){x^3} – 4{x^2} – 9x = – 36\\
\Leftrightarrow {x^3} – 4{x^2} – 9x + 36 = 0\\
\Leftrightarrow \left( {x – 4} \right)\left( {{x^2} – 9} \right) = 0\\
\Leftrightarrow \left( {x – 4} \right)\left( {x – 3} \right)\left( {x + 3} \right) = 0\\
\Leftrightarrow x = 4;x = 3;x = – 3\\
Vậy\,x = 4;x = 3;x = – 3\\
5){x^2} – 7x + 6 = 0\\
\Leftrightarrow \left( {x – 1} \right)\left( {x – 6} \right) = 0\\
\Leftrightarrow x = 1;x = 6\\
Vậy\,x = 1;x = 6\\
6)8{x^3} – 12{x^2} + 6x – 1 = 0\\
\Leftrightarrow {\left( {2x – 1} \right)^3} = 0\\
\Leftrightarrow 2x – 1 = 0\\
\Leftrightarrow x = \dfrac{1}{2}\\
Vậy\,x = \dfrac{1}{2}\\
7){x^2} + 5x + 4 = 0\\
\Leftrightarrow \left( {x + 1} \right)\left( {x + 4} \right) = 0\\
\Leftrightarrow x = – 1;x = – 4\\
Vậy\,x = – 1;x = – 4
\end{array}$